This invention relates to telephone systems and, in particular, to telephone systems employing end offices having end office electronic digital switches.
In present day telephone systems, a local exchange or regional telephone company operating in a local access transport area (i.e., a so-called "LATA") employs central offices having electronic digital switches. These central offices are adapted to process local calls, i.e., calls which originate and are to terminate within the local calling area of the local exchange or regional telephone company, intra-LATA calls, i.e., calls which originate and are to terminate in the "LATA" of the local exchange or regional telephone company, and inter-LATA calls, i.e., calls which either originate or are to terminate outside the LATA of the local exchange or regional telephone company.
Each central office of a local exchange or regional telephone company is usually classified as either an end office, a local tandem office or an access tandem or LATA tandem office. An end office interfaces with subscriber locations within its wire center area as well as with other central offices. These other central offices can be other end offices, local tandem and/or access or LATA tandem offices within the LATA of the local exchange or regional telephone company or central toll offices of an interexchange carrier or long distance company.
Local tandem offices connect end offices within the same local exchange area. Access tandem offices operated by regional telephone companies connect end offices of different local exchange areas within the same LATA of the regional telephone company. LATA tandem offices provide similar end office connections as acces tandem offices, but have the added ability to provide connections to central toll offices of an interexchange carrier.
An end office interfaces with other central offices via trunks which are connected to the trunk side of the electronic digital switch at the end office. Trunks can be classified in terms of the central offices to which they connect their respective end office. Local tandem trunks connect the end office to a local tandem office. Direct local trunks and direct toll trunks connect their respective end office to another end office and to a central toll office of an interexchange carrier, respectively. Toll tandem trunks, on the other hand, connect an end office to an access or LATA tandem office and, in the case of a LATA tandem office, may be shared by inter and intra-LATA toll calls.
An end office interfaces with subscriber locations via line side equipment which connects to the line side of the end office electronic digital switch. The line side equipment includes a time slot interchanger and a line circuit for each subscriber location. The line side equipment also includes circuitry for reducing the level of echo signals present on the transmit paths included in the line side equipment for each subscriber location.
The echo signals which are unavoidably present in each transmit path are primarily echoes of the far end party's voice signals. These echo signals are carried to the receive path of the line side equipment serving the far end party and are reduced there based upon a so-called fixed loss plan conventionally used by each regional telephone company. The fixed loss plan provides that a predetermined fixed amount of loss be present in a receive path. The particular amount of fixed loss depends upon whether the call is intra-office, intra- exchange (local), intra-LATA (toll) or inter-LATA (toll).
The fixed loss plan requires that a 0 dB loss be present in the receive path for an intra-office call, i.e., a call within the same wire center area served by the end office. For calls continued directly to another end office (i.e., intra-exchange calls), the fixed loss plan requires that a 3 dB loss be present in the receive path. For all other calls, i.e., those continued through local, access or LATA tandem offices or directly through central toll offices of an interexchange carrier, the fixed loss plan provides that a 6 dB loss be introduced into the receive path.
In order to provide the required 3 dB or 6 dB loss in each receive path, the line side equipment at each end office inserts either analog loss in the corresponding line circuit or digital loss in the corresponding digital time slot interchanger, or a combination of both. This loss is inserted into the receive path under the control of the digital switch of the end office.
More particularly, the end office digital switch, determines from the type of trunk circuit seized and from a stored fixed loss plan table correlating trunk types to required fixed loss, whether a 0, 3 dB or 6 dB loss should be inserted into the corresponding receive path. The digital switch software then controls either the line circuit or the time slot interchanger, or both, to provide the required loss in the receive path. This might require removal and/or insertion of loss or maintaining the status quo depending upon the existing loss condition of the path.
As above-indicated, the line side equipment at each end office also includes circuitry for reducing echo signals coupled to each of the transmit paths in the line side equipment. These echo signals are those of the far end party and, as above-noted, are unavoidably returned in some part to the far end party via the near end party's transmit line. The circuitry for this is included in the hybrid circuit of the line circuit and is provided by a variable impedance.
The variable impedance of the hybrid circuit is controlled by the end office digital switch. The switch adjusts this impedance so as to more nearly match the impedance seen by signals forwarded to a subscriber line. This effectively reduces the presence of far end echo signals on the transmit path.
The use of the above-described fixed loss plan and variable impedance hybrid in each end office of a local exchange or regional telephone company has proven successful in realizing acceptable levels for the echo signals. However, the fixed loss plan has certain apparent disadvantages. One disadvantage is the cost and complexity of implementing and maintaining the plan. This is manifested in the need to include loss in each line circuit and/or time slot interchanger in the line side equipment. It is further manifested in the need to adapt the end office digital switch so that it can now control insertion of the required loss in the receive paths.
Another disadvantage is that the presence of the loss in the receive paths reduces the amplitude level of the received signals. As a result, call signals are not received at the optimum amplitude level for hearing. Also, any noise, whether on the receive circuit or in the background of the receiver becomes more bothersome, since the receive signal is lower. Additionally, when digital loss is inserted via the time slot interchanger, analog data signals carried over the same voice path, are impaired by quatification of the digital loss inserted.
It is, therefore, an object of the present invention to provide an end office and method for a telephone system of the above type in which the amplitude level of calls on the receive paths in the line side equipment is improved toward optimization.
It is a further object of the present invention to provide an end office and method which satisfy the above objective in a way which reduces the complexity and amount of equipment or software required to be used in the end office.